Customary, capacitively coupled RF-plasma reactors comprise first and second mutually spaced electrodes which concomitantly confine the plasma reaction volume. Besides of the two electrodes, their electrode conductive surfaces, no further electroconductive parts act as a third electrode with an externally applied electrical potential into the plasma reaction volume. Therefore, typical capacitively coupled RF-plasma reactors are also said "diode type" reactors. Confining the plasma reaction volume between the two electrodes which are mutually driven at electric RF potential is normally achieved in providing a spacing at the border of the two electrodes which suffices for electric isolation in terms of DC, but which prevents spreading of the plasma discharge outside the space confined by the two electrodes. This may be achieved e.g. by spacing the two electrodes at their periphery by a distance smaller than dark space distance at the respective operating conditions.
Such capacitively coupled RF-plasma reactors have been used for many years in the field of plasma processing, especially for treating flat substrates such as silicon wafers.
The moat common applications of such reactors are Plasma-Enhanced Chemical Vapour Deposition (PECVD) and Plasma Dry Etching. Plasma Dry Etching can be divided into Reactive Ion Etching (RIE), in which ion bombardment is used to promote an anisotropic etching, or in Plasma Etching (PE), in which ion bombardment is to be avoided.
Plasma cleaning, removal of a polymer resist (Ashing) or plasma induced surface activation of workpieces can also be performed in a capacitively coupled RF plasma reactor. Most such reactors are construed with planar electrodes.
In these applications the plasma is generated by a driving RF voltage, most commonly for industrial applications at a frequency of 13.56 MHz.
An RF-driven plasma develops a large self bias voltage between the plasma and the plasma reaction volume surrounding walls which are, as was stated, the electrode surfaces. This is due to the rectifying effect of the RF voltage across the plasma sheath by the plasma sheath according to the dark space adjacent to each of the electrode surfaces towards the plasma discharge or reaction volume. As a consequence, in a classical capacitively coupled RF reactor the ion bombardment of the workpiece or substrate is governed by the ratio between the two electrode surface areas which are in contact with the plasma discharge. Most plasma self bias, enhancing ion bombardment, arises adjacent to the electrode surface of the smaller electrode area, and the potential difference across the sheath or dark-space adjacent to the larger area electrode is accordingly smaller. This effect is conveniently exploited by plasma process designers to taylor the ion bombardment on the substrate. E.g. for RIE the workpiece or substrate is placed adjacent or on the smaller-area electrode coupled to the plasma which results in larger ion bombardment. Inversively, for PE the workpiece is placed adjacent or on the larger-area electrode where ion bombardment is smaller.
There is today a need for plasma processing of large surface workpieces as e.g. of large surface glass plates. E.g. the flat display industry is today considering manufacturing on such plates of about 1 m.sup.2. The type of processes to be performed to produce the pixel related electronic circuits on such glass plates are of the same nature as the processes used in micro-electronics, but have to be performed on significantly larger surface workpieces.
If a large surface substrate is to be processed by a capacitively coupled RF plasma reactor and at process conditions similar to those as used when treating silicon wafers, the plasma gap (the distance between the two electrodes, e.g. of a planar capacitive reactor) must be kept at the predetermined value in order to achieve the number of collisions for exited species as desired. Thus, a typical value for plasma gap is between 1 and 10 cm (both limits included).
With the large surface workpieces and such plasma gaps the electrodes of the reactor take a very extended aspect ratio and the electrode surface area ratio becomes close to 1. This implies that in a capacitively coupled RF plasma reactor for large surface workpieces there is almost no control of ion bombardment. The ion bombardment occurs substantially equally at both electrode surfaces. Such a situation is strongly limiting processing large-size workpieces and especially large-size substrates by known capacitively coupled RF plasma processes as they were developped for the micro-electronic industry.